Potassium tantalate niobate crystal growth from a melt



CRYSTAh This improvement relates to a discovery of an improved methodfor producing improved large, mixed K'RN (KTa Nb O potassium tantalateniobate single crystals wherein l X 0, and the product obtained thereby.More particularly, the improvement concerns an improved method ofsynthesis of large, homogeneous single KTN crystals of improvedelectrical parameters from selected compositions of K(Ta,Nb)O preferablywithin the range of KTa Nb O and having controlled properties relatingto composition, size, Curie temperature, dielectric constant, andoptical clarity, and the products thereof.

In general, it is known that KTN is a mixed crystal of two perovskites,KTaO and KnbO which may be combined in almost any proportion by usingknown reducing temperature crystal forming technique, to grow crystalsof small size and varying composition, dependent upon the changingsolution concentration.

As illustrated in Triebwasser Patent No. 2,954,300, a phase equilibriumdiagram for the system KNbO KTaQ' is also known to the art and used withreference to the general knowledge of crystal growth from cooling andslow evaporation of a saturated solution applying the Kyropoulostechnique. Yet the surprising fact remains that the art is dependentupon non-homogeneous KTN crystals of small size and having a largemeasure of optical interference. Thus, reducing their value inelectrooptical devices.

Accordingly, it is an object of this disclosure to provide an improvedmethod of synthesizing crystals of the KTN type (KTa Nb O and On theorder of KTa Nb O or X is on the order of .30.70, having a crystal sizeon the order of 30 x 20 x mm. to 40 x 40 x 30 mm. and weighing on theorder of 34 grams to 142 grams.

Another object of this disclosure is to provide the art with thediscovery of an improved and steady state method of synthesizing agreater and improved and more consistent yield of useful optical qualityhomogeneous single large crystals of KTN(KNbO KTaO and particularly KTaNb O per size of melt, having a Curie temperature from 52 C. to +16 C.,i3 C., a crystal size in excess of 30 x 20 x 5 mm., a crystal weight upto and about 142 grams of improved optical quality of reduced veilingand controlled electrical properties, and the product obtained thereby.

Further objects and advantages will be apparent from the followingdescription in relationship to the accompanying drawing wherein:

FIG. 1 is illustrative of using the linear function of Curie temperaturerelative to crystal composition as herein contemplated; and

FIG. 2 is illustrative of selective melt compositions used todemonstrate optimum conditions to obtain optimum results as hereinprovided.

Additional objects and advantages will be recognized from the followingdescription wherein the examples are given for purposes of illustration.To the, accomplishment of the foregoing and related ends, this inventionthen comprises the features hereinafter more fully described andinherent therein, and as particularly pointed out in the claims. Suchillustrative embodiments are indicative of ted States meat the variousways in which the principle of our discovery, invention or improvementsmay be employed.

The method herein and herewith provided can be used for moreconsistently growing large homogeneous single KTN crystals of a size upto and about 142' grams of the mixed composition type of KTa Nb O w,here1 X 0 and X is preferably on the order of .60 to .70. In general, themethod provides for more uniform fKTN crystal growth at a constanttemperature utilizing a steady state condition to yield largehomogeneous crystals of improved high optical quality useful inelectro-optical modulation of electromagnetic radiation. .1

With reference to FIG. 1, the composition applicable to the desiredCurie temperature is determined by using the linear function of Curietemperaturen versus molar composition of KTa Nb O To select thecomposition of the melt 1 from which to produce the crystal, referenceismade to the phase equilibrium diagram of FIG. 2. For example, aselected composition being KTa Nb O as evidenced by the composition linedc, extends upwardly until it intersects the solidus curve 0.; Anisotherm line is drawn from c which intersects the liquidus curve at a.This represents the composition of the liquid in equilibrium with thesolid of composition e. The choice of this melt composition lies alongthe line ac. Thereby, it will be evident that two factors are used todetermine the optimum composition of the melt: (1), the ratio of solidin equilibrium with liquid represented on the diagram by the lengthratio of abzbc must be l to obtain the largest yield from the method;(2) since the melt will contain a solid phase in equilibrium with aliquid phase, the final composition must be selected after experimentaltrial to realize how much solid in the melt will interfere physicallywith ionic movement and with the growing crystal.

Illustrative of the method herein provided, a powdered composition ofKTa Nb O in stoichiometric proportion, was mixed and added to a platinumcrucible of 200 cc. capacity with excess K CO to the extent that amountof K 0 excess was 15 mol percent compared to stoichiometry. Whereas K COis shown to work alone, it will be recognized that a small amount ofadditional material as SnO or the like, may be added to aid instabilizing the pentavalent state of niobium, if desired. Further, therate of crystal growth appears to be controlled by the amount of excessK 0 in the melt and where economically feasible, permitting slowergrowth by longer soak time, a reduced amount of such agent ispermissible with even less lamellae and minimum optical interference.Initially, the mixture is raised to a melt temperature T above theliquidus curve, forming a complete liquid phase to ensure completemixing of the materials and enable the addition of more crystal formingmaterial mixed in powder form, in proportions as described previously,to raise the level of the melt and filling the crucible, or filling thecrucible to a predetermined level. The mixture is then placed (or themixture affected) in a furnace zone of constant temperature andmaintained at T or in liquid form, preferably with a flow of oxygenmaintained past the melt, about the crucible, or from the base andthrough the furnace during crystal growth. The temperature is thenlowered to T (growth temperature) or melt and a small crystal seed ofKTN composition, strung on a platinum wire, is immersed approximately 5mm. below the surface of the melt maintained at a uniform melt temperature, at T above 1200 C., in this instance. The seed is rotatedslowly at a rate of about 8 rpm. with the melt temperature held constantor in a steady state within 1 C. of the melt temperature. Rotation ofthe crucible 1 Melt as used herein throughout the application refers toan equilibrium mixture of liquid and solid for a given chargecomposition at a given temperature.

may be affected, if desired. However, under the constant statecondition, as herein prmided, the usually undesirable vertical thermalgradient is of advantage as normal convection currents circulatematerial from the bottom of the melt to the top where crystal growthoccurs. The transfer of material occurs, not only through thermalgradient phenomena but homogeneity at the growth stage is maintainedQbyionic movement caused by concentration gradients in the melt, makingrotation of the melt container unnecessary.

square in cross section and had clearl designated simple cube faces.

By considering the phase equilibrium diagram in relationship to theCurie temperature and growing crystals, by the above process, largecrystals were grown in composition range on the order of from KTa Nb Oto KTa Nb O the Curie temperature varied within :3? C. within a range of+16 C. to -52 C., and havingother advantageous properties, evidenced inthe following table testing 5 mm. cubes cut therefrom,

TABLE I Crystal Dimensions, Weight Resistivity Avg. Retardation,composition mm. (g.) m-crn. Tei3 C. voltage KTaMNbMO; 30 x 30 x 5 345X10 -19 V:=2,810 V41=3,970 Ve=4,870 30 x 30 x 5 34 5X10" 45 Vh=3,500 30x 26 X 7. 43 -52 35 x 35 x 101 9X10 +10 Vaf=700 KT8 so Nb.an7O3 40 X 40X 142 8X10 9 +15 V =700 KTfijflNbjHOQ 30 x x 12 46 5X10 11 1 V;,=1,400KTa wNb,4oO3 45 x 45 x 10 129 2X10 5 +16 Vn=700 In lowering from T to Tthe T temperature is passed at which solid c' is in equilibrium withliquid b. Crystals may precipitate out in a range of compositionfollowing the solidus curve from c" to c. However, this is not anequilibrium condition and the crystals react with the melt providing newmaterial for equilibrium of seed crystal growth. Crystal growthcontinues until either of two con ditions exist: 1) equilibrium isreached and growth stops or (2) the level of the liquid in the cruciblefalls too low. Before the level falls too low and/or just about when thegrowth stops, as in conditions (1) and (2), the grown crystal iswithdrawn to a position slightly above the melt and cooled down, asalong the line cd of FIG. 2, at the rate of approximately 15 to 20 C.per hours, to room temperature.

In the above process, no cooling of the melt is per mitted duringcrystal growth for the production of high quality, homogeneous crystalsof the type KTa Nb O as herein described. As will be recognized, theproper size and selection of an optimum melt composition is empiricalrelative to the crystal desired per melt batch. However, by properselection of a melt composition in the range indicated, a largehomogeneous single crystal can be grown under the steady statecondition, as described.

As an alternative use of the above process, the growth of KTN crystalscan be affected by use of Nb O Ta O and K CO mixed in stoichiometricproportion with 15 mol percent excess K 0 (using K CO The mixture, inpowder form was used to fill a 200 cm. platinum crucible and melted downto a liquid state. While maintained in the melt state, as described, aseed crystal of KTaO (KTN) was suspended on a platinum wire strungthrough an aperture therein and the KTN crystal grown as in the aboveprocess. The desired growth temperature was determined by calibratingwith the Curie temperature. That is, the growth temperature ismaintained substantially constant to :1 C. in the range of over andabove 1100 C., or at melt temperature, with the seed remaining in themelt mix for a period of from about four to five days. When growth'stops or the melt mix falls too low so as to change crystal composition,the grown crystal is lifted out of the melt and cooled to roomtemperature. By this method crystals with dimensions up to 40 X 40 x 30mm. and weighing up to 142 grams have been grown in a period of fourdays time.

By the above method of crystal growth, homogeneous crystals of highoptical quality with dimensions of from 30 x 30 x 5 mm. to 40 x 40 x 30mm. and weighing as much as 142 grams were grown in periods of four tofive days. Using an (001) seed, the crystals grew generally Bycomparison with known standard crystal material, the size differentialprovided by the herein described progess is discovered to afford agreater yield of optically usable material per run with the advantage ofimproved optical quality, thus providing a greater selectivity of moreuseful optical crystals per melt. That is, it has been discovered thatthe method provided herein consistently yields larger crystals of morehomogeneity of composition with improved optical quality, electricalparameters and wide band gap.

Having described the present embodiments of our improvement is in theart in accordance with the patent statutes, it will be apparent thatsome modifications and variations may be made without departing from thespirit and scope thereof. The specific embodiments described are givenby way of examples illustrative of our discovery. invention orimprovement which is to be limited only by the terms of the appendedclaims.

What is claimed is:

1. The method of growing a homogeneous KTN crystal weighing onthe orderof from 34 grams to 142 grams of a size ranging on the order of from 30x 30 x 5 to 40 x 40 x 30 mm., having a Curie temperature within a rangeof +16 C. to 52 C., :3" C., and high optical quality comprising thesteps:

(1). preparing a crystal forming liquid mixture of KTa Nb O and 1 X 0 inthe presence of K 0 at crystal melting temperature;

(2 reducing the temperature of said mixture and maintaining the saidmixture at a substantially constant temperature of equilibrium mixtureof liquid and solid;

(3) suspending a seed crystal of KTN composition about 5 mm. below thesurface of said mixture;

(4) maintaining the said mixture at a substantially constant temperaturethroughout crystal growth and with the seed crystal suspended therein; 1

(5) effecting the defined crystal growth for a period of up to aboutfour to five days;

(6) with drawing the grown crystal from the said mixture and coolingsaid crystal to room temperature.

2. The method of claim 1 wherein the selected composition of the solidportion of the melt KTa Nb O is on the order of KTa Nb O and the K 0 isin stoichiometric excess up to about 15 3. The method of claim 1 whereinthe selected composition of KTa Nb O of the melt mix is on the order ofKTa Nb O and the K 0 is in stoichiometric excess up to about 15%.

4. The method of claim 1 wherein the selected com- 5 position of KTa NhO of the melt mix is on the order of KTa Nb O and the K 0 is instoichiometric ex cess up to about 15 5. The method of claim 1 whereinthe selected com position of KTa Nb O of the melt mix is onthe order OfKTa.3o -1o and Nb.30 "7oo3 and the K30 is StoiChiometric excess up toabout 15%,

6. The process of claim 1 including the step of maintaining a flow ofoxygen past said mixture.

7. The method of claim 1 including the step of slowly rotating the seedcrystal'in said liquid mix during the formation of said crystal.

8. The method of claim 1 includingthe step of providing a flow of oxygenpast said liquid mix during crystal growth,

References Cited UNITED STATES PATENTS 2,758,008 8/1956 Reisman et a1.um- 23 21 3,065,046 11/1962 FOOS 23 51 5 3,092,448 6/1963 Kennedy 23 -153,129,061 4/1964 Dermatis et a1. 23-501 3,244,488 4/1966 Linares et a1.23-301 3,346,344 10/1967 Levinstein et a1. 23- 301 10 3,366,510 1/1968Daendliker M2 23-s1 NORMAN YUDKOFF, Primary Examiner,

U.S.C1.X.R.

1. THE METHOD OF GROWING A HOMOGENEOUS KTN CRYSTAL WEIGHING ON THE ORDER OF FROM 34 GRAMS TO 142 GRAMS OF A SIZE RANGING ON THE ORDER OF FROM 30 X 30 X 5 MM. TO 40 X 40 X 30 MM., HAVING A CURIE TEMPERATURE WITHIN A RANGE OF +16* C. TO -52* C., $3*C., AND HIGH OPTICAL QUALITY COMPRISING THE STEPS: (1) PREPARING A CRYSTAL FORMING LIQUID MIXTURE OF KTAXNB1-XO3 AND 1>X>0 IN THE PRESENCE OF K2O AT CRYSTAL MELTING TEMPERATURE; (2) REDUCING THE TEMPERATURE OF SAID MIXTURE AND MAINTAINING THE SAID MIXTURE AT A SUBSTANTIALLY CONSTANT TEMPERATURE OF EQUILIBRIUM MIXTURE OF LIQUID AND SOLID; (3) SUSPENDING A SEED CRYSTAL OF KTN COMPOSITION ABOUT 5 MM. BELOW THE SURFACE OF SAID MIXTURE; (4) MAINTAINING THE SAID MIXTURE AT A SUBSTANTIALLY CONSTANT TEMPERATURE THROUGHOUT CRYSTAL GROWTH AND WITH THE SEED CRYSTAL SUSPENDED THEREIN; (5) EFFECTING THE DEFINED CRYSTAL GROWTH FOR A PERIOD OF ABOUT FOUR TO FIVE DAYS; (6) WITH DRAWING THE GROWN CRYSTAL FROM THE SAID MIXTURE AND COOLING SAID CRYSTAL TO ROOM TEMPERATURE. 